U.S. Pat. No. 6,951,685 issued Oct. 4, 2005 describes a method for the manufacture of ultra high molecular weight polyethylene (UHMWPE) materials in the form of slit film fibers, tapes and narrow sheets. Such materials are described as having, among other useful properties ballistic resistance of a very high order.
As is apparent to the skilled artisan, the equipment and processing techniques described in this patent require significant capital investment and the application of relatively stringent processing conditions. Both of these requirements increase virtually exponentially as the UHMWPE product width is increased from a fiber to a tape and upwards to a sheet. Thus, in order to contain the additional cost of equipment required to make such wider materials, i.e. sheet as defined herein, it would be desirable to have a method for their manufacture that minimizes such costs and process control requirements.
U.S. Pat. No. 4,931,126 describes an apparatus for increasing the width of a fiber reinforced thermoplastic sheet or tape product, which apparatus increases such width by longitudinally joining parallel sheets or tapes of the fiber reinforced thermoplastic material in an overlap or butt configuration and melting the overlapping or abutting areas of the parallel tapes.
European Patent Publication No. EP 1 627 719 A1 describes a multilayered UHMWPE material comprising a plurality of “monolayers” of UHMWPE in the absence of any adhesive wherein the each monolayer is laid at an angle to any adjacent monolayer. The term “monolayer” as used in this publication is defined as comprising “a plurality of high-strength unidirectional polyethylene strips, oriented in parallel in one plane, next to one another”. According to one embodiment the strips partially overlap. The “monolayers” are formed by subjecting the overlying strips to conditions of temperature and pressure in the ranges of 110-150° C. and 10-100 N/cm2 These conditions produce a “sheet” having joint areas that are inadequate to maintain even a modicum of integrity and their properties are grossly inferior to those of the sheets of the present invention, as will be demonstrated in the discussion and examples that follow.
There thus remains a need for a method of producing wide strips or sheets of substantially pure and highly oriented UHMWPE from narrower tapes or strips of these materials, and for the products produced by such a method.